Polyethylene (PE) is synthesized by polymerizing ethylene (CH2═CH2) monomers. Because it is cheap, safe, stable to most environments and easy to be processed, polyethylene polymers are useful in many applications. According to the properties, polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). In another classification, the used polyethylene can be classified as Ultra High Molecular Weight (UHMW), High Molecular Weight (HMW), Medium Molecular Weight (MMW) and Low Molecular Weight (LMW). Each type of polyethylene has different properties and characteristics.
Ethylene polymerization processes are frequently carried out in a loop reactor using ethylene monomer, liquid diluent and catalyst, optionally one or more co-monomer(s), optionally an activating agent or co-catalyst and optionally hydrogen. The polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in the form of solid particles which are suspended in the diluent. The slurry in the reactor is circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash tank, where most of the diluent and unreacted monomers are flashed off and recycled.
After the polymer product is collected from the reactor and the hydrocarbon residues are removed, the polymer product is dried, additives can be added and finally the polymer may be mixed and pelletized. The resulting product can then be used for the manufacturing of various objects.
The molecular weight distribution is of great influence for polymer processing. Polyethylenes with narrow molecular weight distribution lead to products with greater dimensional stability, higher impact resistance, and higher resistance to environment stress-cracking as compared to those having broad molecular weight distribution. On the other hand, polyethylenes having broad molecular weight distribution show greater flowability in the molten state at high shear rate, which is important for blowing and extraction techniques. Thus, there is a need to control the molecular weight distribution of polyethylenes.